Pre-charging 12v battery?
 

Hi there,

Have an '03 Honda Element (with Honda's two-stage alternator system). Been looking at what we can do WRT alternator disabling for a road trip car, but as a small easy thing - would charging the 12v battery via a grid charger before every trip reduce the load on the alternator // improve FE? We'd be happy to just plug in the Element before/after each drive if so!

Thx,

You wouldn't want to disable the alternator on a road trip. Alternator disabling is best for shorter trips where you won't discharge the battery too much.

Someone else might know if your alternator can be disabled with something as simple as a switch, but my understanding is many of them aren't so easily disabled, so people trick it into thinking the battery is full by sending a ~15v signal to it.

Topping off the battery with a charger will help, but it won't help enough for you to see the fuel savings.

Redpoint is correct. The alternator keeps the battery topped off to the point that a little charging isn't going to make any significant change in fuel economy. If you want to disable your alternator, you need to swap in a deep cycle lead acid or other chemistry battery that has enough capacity for the trips you'll be taking.

I had a tender installed under the hood of my last car, and I had it on a multi-outlet with the block and oil pan heater, so it would top up the 12v before leaving. I never tried to isolate the gains (if there were any) but I have no doubts it would disappear into background noise. I simply had the tender, so I used it.

I say, why not try it?

The things to keep in mind are:

• How much is really lost to the alternator? Some say 10%. But that includes the friction of the alternator spinning. To completely get rid of the alternator's drag, you have to get rid of the alternator completely.
• Long trips will use more electricity. But do we know how much? Are these going to be during the day or during the night? How far are we talking? Will you be running accessories like A/C or heater? Watts x hours = watthours. If you can reduce your electrical use you can drive farther on one charged battery. Of course if you reduce your electrical use you also don't benefit as much from this mod.
• Battery size needs to be big enough to fulfill its purpose. But bigger batteries are more expensive.
• Lead acid batteries last the longest if you keep them topped off. Run them dead and you can lose 50% of their capacity just like that.

Lead acid batteries are very inefficient when topping off, so if you are paying for that energy, you will easily and up losing money.

On an 03 Element with lead acid battery, what others said.

I just put a small solar panel on my new CR-V to keep it topped off because I noticed the start-stop doesn't work if the battery isn't at maximum capacity, and because I don't drive it much.

If you want to save the small amount of power that the car consumes while sitting, maybe consider a small 3 dollar solar panel? It's very miniscule in terms of fuel, but nicer to the battery if the car sits a lot.

I also change lead acid batteries at about every 8 or 9 years. The Avalon's 12V batteries cost me some $300. The longer they last the better, even if they are inefficient keeping topped off.

300??? How are they so expensive? That's lithium ion price.

I paid some $320 for them because the only ones I could find are the original Panasonic lead acid batteries from the dealer. None of the parts stores sell batteries that will fit my car.

Lithium would be nice, but I live where it gets way below zero in the winter, so that would freeze and ruin the lithium batteries unless I got lithium titanate batteries, which would be more expensive and lower voltage.